This invention relates to pressure transducers, and more particularly to transducers capable of indicating both pressure magnitude and the crossing of a pressure threshold.
In certain pressure transducer applications it is necessary to read the absolute pressure magnitude only when the pressure has crossed a threshold level and moved into a dangerous or undesirable region. For example, telephone lines are often surrounded by an outer protective conduit, the interior of which is maintained at a greater than atmospheric pressure. Should any leaks develop in the conduit an outward gas flow is established, rather than a flow in the reverse direction, thereby preventing the transmittal of atmospheric contaminants to the telephone lines. So long as the interior conduit pressure is maintained above a minimum level necessary to protect the lines, the absolute pressure level is not critical. Should the interior pressure fall below that level, however, it is highly desirable that a reading of the absolute pressure level be obtained in order to know the severity of the situation and also to aid in locating the leak.
A popular solution to this problem found in the prior art involves the use of a single purpose pressure transducer (i.e., a transducer having an absolute pressure but not a threshold capability), modified so as to monitor the pressure level and provide a signal when the level falls below a given threshold. The monitoring apparatus typically takes the form of solid state circuitry housed within the transducer and connected with an alarm circuit to a remote sensing station. The circuit monitors the transducer output and produces an appropriate signal when that output crosses the threshold. While such a dual function device can operate successfully, the electronic circuitry adds significantly to the total transducer cost, especially when provision is made in the trnasducer for field adjustment of the threshold level in order to enable standardization of the manufactured product and flexibility of application.